Custom result builder declaration in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using a custom result builder?

Consider the following Swift code that uses a custom result builder named StringListBuilder. What will be printed when print(buildList()) is called?

Swift

import Foundation

@resultBuilder
struct StringListBuilder {
    static func buildBlock(_ components: String...) -> [String] {
        return components
    }
}

func buildList() -> [String] {
    StringListBuilder.buildBlock(
        "apple",
        "banana",
        "cherry"
    )
}

print(buildList())

A ["apple", "banana", "cherry"]

B "applebanana cherry"

C ["apple banana cherry"]

D SyntaxError

💡 Hint

Look at how the buildBlock method collects its string components into an array.

The buildBlock method takes a variable number of String arguments and returns them as an array. So the function buildList() returns an array of strings, which is printed as ["apple", "banana", "cherry"].

❓ Predict Output

intermediate

What error does this Swift custom result builder code produce?

Given this Swift code with a custom result builder, what error will occur when compiling?

Swift

@resultBuilder
struct IntBuilder {
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }
}

func buildSum() -> Int {
    IntBuilder.buildBlock(
        1,
        2,
        "3"
    )
}

print(buildSum())

A Output: 6

B RuntimeError: Cannot add String to Int

C TypeError: Cannot convert value of type 'String' to expected argument type 'Int'

D SyntaxError: Missing colon

💡 Hint

Check the types of the arguments passed to buildBlock.

The buildBlock expects only Int arguments, but a String "3" is passed, causing a compile-time type error.

🧠 Conceptual

advanced

Which option correctly implements a custom result builder that supports optional components?

You want to create a custom result builder that can handle optional strings, including them only if they are non-nil. Which implementation of buildOptional is correct?

A

static func buildOptional(_ component: String?) -> [String] {
    if component != nil { return [component!] } else { return [""] }
}

B

static func buildOptional(_ component: String?) -> [String] {
    component.map { [$0] } ?? []
}

C

static func buildOptional(_ component: String?) -> String {
    component ?? ""
}

D

static func buildOptional(_ component: String?) -> [String] {
    [component!]
}

💡 Hint

Think about safely unwrapping optionals and returning an empty array if nil.

Option B safely unwraps the optional and returns an array with the string if non-nil, or an empty array if nil. This matches the expected behavior for buildOptional in a result builder that produces arrays.

❓ Predict Output

advanced

What is the output of this Swift custom result builder code?

Given this Swift code with a custom result builder, what will be printed when print(buildNumber(condition: true)) is called?

Swift

@resultBuilder
struct NumberBuilder {
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }

    static func buildEither(first component: Int) -> Int {
        component
    }

    static func buildEither(second component: Int) -> Int {
        component
    }
}

func buildNumber(condition: Bool) -> Int {
    NumberBuilder.buildBlock(
        1,
        condition ? NumberBuilder.buildEither(first: 2) : NumberBuilder.buildEither(second: 3),
        4
    )
}

print(buildNumber(condition: true))

A Error: Missing buildOptional method

B Output: 10

C Error: Cannot convert value of type 'Int' to expected argument type '[Int]'

D Output: 7

💡 Hint

The ternary selects the first branch (2) when condition is true. Sum 1 + 2 + 4.

The ternary expression selects NumberBuilder.buildEither(first: 2), returning 2. buildBlock(1, 2, 4) sums to 7 via reduce(0, +).

🚀 Application

expert

How many items are in the resulting array from this custom result builder usage?

Given this Swift code with a custom result builder that supports conditionals and optionals, how many strings are in the array returned by buildMenu(showDessert: true)?

Swift

@resultBuilder
struct MenuBuilder {
    static func buildBlock(_ components: [String]...) -> [String] {
        components.flatMap { $0 }
    }

    static func buildOptional(_ component: [String]?) -> [String] {
        component ?? []
    }

    static func buildEither(first component: [String]) -> [String] {
        component
    }

    static func buildEither(second component: [String]) -> [String] {
        component
    }
}

func buildMenu(showDessert: Bool) -> [String] {
    MenuBuilder.buildBlock(
        ["Salad", "Soup"],
        showDessert ? MenuBuilder.buildEither(first: ["Cake"]) : MenuBuilder.buildEither(second: []),
        MenuBuilder.buildOptional(nil)
    )
}

let menu = buildMenu(showDessert: true)
print(menu.count)

A 3

B 2

C 4

D 1

💡 Hint

Count all strings included by the builder, including conditionals and optionals.

The builder combines ["Salad", "Soup"] plus ["Cake"] because showDessert is true, and buildOptional(nil) returns []. So total items are 3.